This application is based on application No. H10-351283 filed in Japan on Dec. 10, 1998, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an illumination optical system for use in an optical apparatus such as a projection-type image display apparatus, and more particularly to an illumination optical system that makes uniform the polarization plane of the light emitted from a light source.
2. Description of the Prior Art
FIG. 12 shows an example of the construction of a conventional projection-type image display apparatus. This projection-type image display apparatus adopts an illumination method of a separate-pupils type, and is provided with an illumination optical system 90, a cross dichroic prism 98 that serves both to separate and to integrate colors, three reflection-type liquid crystal panels 99R, 99G, and 99B, and a projection optical system 100. Moreover, a total-reflection mirror 101 for directing illumination light to the liquid crystal panels 99R, 99G, and 99B is provided at the pupil position of the projection optical system 100. The illumination optical system 90 is composed of a lamp 91 serving as a light source, a reflector 92, a UV/IR cut filter 93, a concave lens 94, an integrator 95, a polarization separation prism 96, and a half-wave plate 97.
The lamp 91 emits white light having random polarization planes. The reflector 92 reflects the light coming from the lamp 91 in such a way as to form it into a converging beam. The UV/IR cut filter 93 transmits only visible light. The concave lens 94 forms the light coming from the reflector 92 into a parallel beam and directs it to the integrator 95.
The integrator 95 is composed of a first lens array 95a and a second lens array 95b, each having a plurality of lens cells, and the polarization separation prism 96 is disposed between these lens arrays 95a and 95b. The lens cells of the first lens array 95a individually focus the light coming from the concave lens 94 in the vicinity of the corresponding lens cells of the second lens array 95b, and the lens cells of the second lens array 95b individually direct the light passing therethrough to the whole surfaces of the liquid crystal panels 99R, 99G, and 99B.
The polarization separation prism 96 is provided with a polarization separation surface 96a that reflects S-polarized light and transmits P-polarized light and a total-reflection surface 96b. The light having random polarization planes that has passed through the first lens array 95a is then separated into S-polarized light, which is reflected from the polarization separation surface 96a, and P-polarized light, which is transmitted through the polarization separation surface 96a. The P-polarized light is then reflected from the total-reflection surface 96b so as to travel in the same direction as the S-polarized light reflected from the polarization separation surface 96a, and then these two types of light enter adjoining lens cells of the second lens array 95b. The half-wave plate 97 is provided on the lens cells that receive the P-polarized light, and serves to convert the received P-polarized light into S-polarized light. Thus, the whole of the light originating from the illumination optical system 90 is now S-polarized.
The projection optical system 100 is composed of a front unit 100a, a rear unit 100b, and an aperture stop 100c. The pupil of the projection optical system 100 is located between the front unit 100a and the rear unit 100b, where the total reflection mirror 101 is so disposed as to close half of the pupil. The aperture stop 100c is disposed in the vicinity of the total-reflection mirror 101. The cross dichroic prism 98 has a dichroic surface 98R that selectively reflects red (R) light and a dichroic surface 98B that selectively reflects blue (B) light, and the liquid crystal panels 99R, 99G, and 99B are so arranged as to face the cross dichroic prism 98 each from a different direction.
The illumination light L1 coming from the illumination optical system 90 is reflected from the total-reflection mirror 101, then travels through the rear unit 100b to enter the cross dichroic prism 98, and is then separated into R light, which is reflected from the dichroic surface 98R, B light, which is reflected from the dichroic surface 98B, and green (G) light, which is transmitted through the dichroic surfaces 98R and 98B. The thus separated R, G, and B light illuminates the liquid crystal panels 99R, 99G, and 99B, respectively, and is reflected therefrom; meanwhile, the R, G, and B light is modulated by the corresponding liquid crystal panels 99R, 99G, and 99B in accordance with the light components of the corresponding colors of the image to be displayed.
The R, G, and B light reflected from and thereby modulated by the liquid crystal panels 99R, 99G, and 99B then enters the cross dichroic prism 98 again, and is integrated together by being reflected by or transmitted through the dichroic surfaces 98R and 98B so as to be formed into projection light L2. The projection light L2 travels along a path symmetrical with the path of the illumination light Li with respect to the optical axis of the projection optical system 100, and is then projected through the projection optical system 100 with enlargement. The projection optical system 100 focuses the projection light L2 on a screen (not shown) and thereby displays a color image thereon.
In this projection-type image display apparatus, the whole of the illumination light L1 is S-polarized when it enters the cross dichroic prism 98. Now, suppose that the cutoff wavelength, at which the transmittance of the cross dichroic prism 98 for S-polarized light equals to 50% is, for example, 580 nm on the dichroic surface 98R and 510 nm on the dichroic surface 98B. Then, the G light that illuminates the liquid crystal panel 99G covers a wavelength range from 510 to 580 nm, and its energy at wavelengths 510 and 580 nm is 50% of the energy it has before entering the cross dichroic prism 98.
The G light, after being reflected from the liquid crystal panel 99G so as to be formed into the projection light L2, passes through the dichroic surfaces 98R and 98B again. Here again, only 50% of the light having wavelengths of 510 and 580 nm is transmitted, and therefore the projection light, when it reaches the screen, has only 25% of its original energy at those wavelengths. Thus, the wavelength range of the G light included in the projection light is narrowed down to, for example, from 520 to 570 nm, within which the transmittance on the dichroic surfaces 98R and 98B is 70% or more (i.e. 50% or more on a two-way basis).
The same is true with the R and B light. Specifically, whereas the wavelength ranges of the illumination light L depend on the wavelengths at which the reflectance on the dichroic surfaces 98R and 98B equals to 50% (i.e. the R light covers a wavelength range from 580 nm and above, and the B light covers a wavelength range from 510 nm and below), the wavelength ranges of the projection light L2 depend on the wavelengths at which the reflectance on the dichroic surfaces 98R and 98B equals to 70% (i.e. 50% on a two-way basis); for example, the R light covers a wavelength range from 590 nm and above, and the B light covers a wavelength range from 500 nm and below.
The specific values given above are simply estimates obtained for principal rays. In general, the characteristics of a dichroic surface depend on the angle of incidence of rays, and the transmittance and reflectance for rays incident on the dichroic surface at different angles from principal rays vary from the transmittance and reflectance for principal rays. If such variation is taken into consideration, the wavelength ranges of the R, G, and B light are narrowed down further.
Thus, the projection-type image display apparatus described above suffers from loss of the energy of light, i.e. loss of the amount of light, around the boundary wavelengths for color separation, i.e. in the wavelength ranges around the cutoff wavelengths. As shown in FIG. 13, the light thus lost becomes stray light L3 that repeats reflection and transmission in and around the cross dichroic prism 98. This stray light L3 may appear as a ghost or act to lower contrast, degrading the quality of the image displayed.
An object of the present invention is to provide an illumination optical system that supplies illumination light that does not cause loss of the amount of light when subjected to color separation and color integration, and to provide a projection-type image display apparatus that employs such an illumination optical system and that thus offers bright and high-quality images.
To achieve the above object, according to one aspect of the present invention, an illumination optical system is provided with: a polarization separation device for separating white light emitted from a light source and having random polarization planes into a first type of light and a second type of light polarized on different planes from each other and traveling in different directions from each other; a polarization plane conversion device for rotating the polarization plane of light of a particular wavelength range included in the first and second types of light; a convergence optical system for making the first and second types of light converge on first and second convergence positions, respectively; and a half-wave plate, disposed near one of the first and second convergence positions, for rotating the polarization plane of the first or second type of light that converges on that convergence position.
According to another aspect of the present invention, a projection-type image display apparatus is provided with: an illumination optical system as described above; a display device for displaying an image in accordance with image data fed thereto so as to modulate light in accordance with the image thus displayed; a first optical system for directing the light output from the illumination optical system to the display device; and a second optical system for projecting the light output from the display device.
According to still another aspect of the present invention, an optical system is provided with: a polarization device for polarizing white light emitted from a light source and having random polarization planes in such a way as to convert the light into light having a predetermined polarization plane; and a polarization plane conversion device for converting the polarization plane of light of a particular wavelength range included in the light polarized by the polarization device in such a way that the light of the particular wavelength range has a different polarization plane from light of other wavelength ranges.